This invention relates to synthetic polymer emulsion, and particularly, this invention relates to a composition comprising mixed synthetic polymer emulsion which possesses good film forming property.
The film forming mechanism of various kinds of synthetic polymer emulsion has been widely studied, and it is known that the film formation of polymer emulsion occurs the following process. Under the temperature higher than glass transition temperature, polymer particles change their shape and coalesce each other by vaporizing water contained in emulsion. But in order to make perfect film formation, this process needs at least a few days. Under higher temperature than glass transition temperature and short time drying like industrial process, film formation partially occurs and does not give their good properties. Difficulty of short time drying causes limited various applications of synthetic polymer emulsion.
In order to solve the above problem, coalescent agents those are vaporizable solvent existing in water or polymer particles are used to make glass transition temperature (hereinafter referred to as Tg) of polymer lower temporarily. However, this method is not suitable for air pollution problem.
To improve film formation, fine particle emulsion polymers those have strong capillary pressure in coalescence are commonly used. But it needs much surfactant to produce such fine particle emulsion. Generally using much surfactant is incident to produce polymer having poor water resistance.
Another method to improve film formation is using the mixture of higher Tg polymer emulsion and lower Tg polymer emulsion. But the mixture has sharp change of film forming temperature and must contain much lower Tg polymer emulsion making their film tacky to keep enough film formation. Such tackiness may happen blocking problem of products.
As the result, it is very difficult to get polymer emulsion or compounds those have higher Tg, good film formation at low temperature or short time drying and good blocking resistance.
Especially preventing moisture transparent coatings for pulp paper or sheet that needs good film formation under the forced drying process during short period and at low temperature is one of the most difficult matters.
In this field, the polymer emulsion containing low Tg polymer with good film forming property, must contain much inorganic fillers or pigments and waxes to prevent blocking.
The present invention is based upon making polymer emulsion having good film forming property under low temperature and/or forced short time dying without vaporizable organic solvents that promotes the change of particle shape and coalescence.
The inventor has conducted extensive researches about coagulations and absorptions concerning emulsion polymer particles.
As the result, the inventor found that the mixtures of emulsion polymers which have specific particle size range and different Tg, adsorb each other and develop good film forming properties. The polymer emulsion is used for making moisture-proof coated paper and/or recyclable-coated paper.
In order to obtain the polymer emulsion of the present invention, stable and fine particle emulsion polymers with Tg of 20 degree (xc2x0 C.) lower than that of the desired emulsion composition should be prepared. The particle size of the emulsion polymers should be 0.10 xcexcm or shorter in diameter, preferably 0.03 to 0.09 xcexcm. Hereinafter, such polymer emulsion is referred to as the fine particle emulsion.
Further, an emulsion containing the polymer of a desired Tg has the particle size of 0.15 xcexcm or longer in diameter and preferably more than 2 times in diameter of the fine particle and improved film forming emulsion is produced by adjusting an amount of an emulsifier in emulsion polymerization. Hereinafter, such emulsion polymer is referred to as the large particle emulsion.
When emulsion polymerization of the large particle emulsion is finished or in proceeding, the fine particle and improved film forming emulsion is added to a reactor for making mixture. In above process for obtaining the polymer emulsion of the present invention, the fine particle portion of the fine particle emulsion may be decreased or disappeared by checking particle distribution. The mixture of the fine particle emulsion and the large particle emulsion may be partially reacted.
In order to obtain the polymer emulsion of the present invention, the particle size of the large particle emulsion is important and should be 0.15 xcexcm or longer in diameter. If emulsion polymerization formula and condition of the large particle emulsion are selected to produce particle size of smaller than 0.15 xcexcm, the emulsions produced are too stable to absorb the fine particle and improved film forming emulsion, and do not have good film forming property. Also the particle size of the fine particle emulsion is important and should be 0.1 xcexcm or shorter in diameter. If the emulsions with the particle size of 0.1 xcexcm or longer in diameter are selected, the emulsions produced cannot absorb the fine particle.
The particle size or diameter means volume median diameter and can be measured by laser scattering particle size analyzer throughout this specification and claims.
In order to obtain the polymer emulsion having good film-forming property, the particle size ratio between particle size of the large particle emulsion and the particle size of the fine particle emulsion is preferably 2 or more. In case the particle size ratio is smaller than 2, each of the large particle emulsion and the fine particle emulsion tends to exist independently. As the result, such mixture may not show improved film-forming property.
In order to obtain the polymer emulsion having good film-forming property, mixture ratio between the solid content of the desired and large particle emulsion and the solid content of the fine particle emulsion is preferably 50:50 to 95:5.
In case the mixture ratio of the fine particle emulsion is larger than 50%, the composition may be the mixture of absorbed particle emulsion and the fine particle emulsion. As the result, the polymer emulsion may have poor blocking resistance. If the mixture ratio is smaller than 5%, the composition may be unstable and may not show improved film-forming property.
Hereinafter, the present invention is mentioned in detail.
The polymer emulsion of the present invention is used for application on various purposes needing good film-forming and blocking resistance by high Tg and good film-forming property.
Synthetic emulsion polymer of the fine particle emulsion or the large particle emulsion generally means emulsion polymer that is made by emulsion polymerization with one or more monomers that have radically reactive groups. Examples of the emulsion polymers include styrene-butadiene emulsion (latex), acrylic emulsion, vinyl-acetate emulsion etc. in the present invention. Monomers that have radically reactive groups are chemical substances that contain vinyl groups in their molecular and are made industrially.
Synthetic emulsion polymer in the present invention is prepared by widely known emulsion polymerization method. There are many published literatures about emulsion polymerization method, for example, xe2x80x9cEmulsion Latex Handbookxe2x80x9d that was published by Taiseisha Co., Ltd. Generally the emulsion polymers are prepared by reacting the radically reactive monomers mentioned above in water medium that contain an emulsifier and a water-soluble radical initiator. Examples of the emulsifier include surfactants such as alkylbenzenesulfonates (e.g. sodium dodecylbenzenesulfonate), dialkyl sulfosuccinates(e.g. sodium dodecylsulfosuccinate) and polyoxyethylene alkylphenyl ether sulfates (e.g. sodium polyoxyethylene alkylphenyl ethoxy sulfate). Examples of the water-soluble radical initiator include persulfates (e.g. ammonium persulfate) and water soluble hydroperoxide(e.g. t-butyl hydroperoxide).
The polymer emulsion of the present invention has good film forming property, and can be produced by mixing the fine particle emulsion and the large particle emulsion.
The fine particle emulsion is the emulsion having particle of 0.1 xcexcm or shorter in diameter. This emulsion is generally produced by polymerization of one or more radically reactive monomers in water medium that contain an emulsifier at 0.5% to 5% by weight based on the total amount of monomer and water-soluble radical initiator. Anionic surfactants those include anionic groups (e.g. sulfonate, sulfate, carboxylate) in molecular are preferably used as the emulsifiers mentioned above.
Monomers those contain ionic groups in molecular is used in emulsion polymerization cooperate with other monomers.
As the fine particle emulsion used in this invention, fine particle emulsions having particle size of smaller than 0.1 xcexcm in diameter from various kinds of emulsion polymer products commercially available that are stable chemically and mechanically and good stability for storage.
The particle size of the polymer in the fine particle emulsion depends on the particle size of the polymer in the large particle emulsion in this invention. The particle size ratio between the large particle emulsion and the fine particle emulsion should be 2 or more. If the ratio is less than 2, the polymer emulsion does not have good film formation property.
The fine particle emulsion comprises polymers having the large particle size of 0.10 xcexcm or shorter in diameter. It usually comprises 30-60% of a polymer, 0.2-6% of an emulsifier and 35-68% of water.
Further, the large particle emulsion comprises polymers having the large particle size of 0.15 xcexcm or longer in diameter. It usually comprises 40-60% of a polymer, 0.1-4% of an emulsifier and 35-58% of water.
The large particle emulsion that has the particle size of 0.15 xcexcm or longer in diameter is prepared by known emulsion polymerization process with a small amount of emulsifiers. Namely, the size of the particle in the emulsion can be adjusted by the amount of the emulsifier, though the kind and the quantity of emulsifiers depend on the conditions of polymerization like temperature, monomer concentration in polymerization, composition of monomer, solid content of product, etc.
When the large particle emulsion is prepared by using a small amount of emulsifiers, the obtained emulsion is generally unstable in storage, chemically and mechanically and does not tolerate in industrial process. It is marvelous that such large particle emulsion obtained in this invention changes to be stable in storage, chemically and mechanically and has good film forming property and also tolerates in industrial process.
The simple method of producing emulsion composition with good film forming property in the present invention is simply mixing the fine particle emulsion with the large particle emulsion. In order to produce more effective emulsion, it is suitable to add the fine particle emulsion to reactor mass of polymerization of the large particle emulsion when polymerization of the large particle emulsion has finished or all most of monomers are converted to polymer.
Also it is suitable to add the fine particle emulsion to reactor mass of polymerization of the large particle emulsion when residual monomer is still existing. In this case, grafting between large particle and fine particle may occur as same as absorption.
In order to obtain the polymer emulsion of the present invention, the mixing ratio between the polymer content of the large particle emulsion and the polymer content of the fine particle emulsion, is preferably 50:50 to 95:5, more preferably 70:30 to 90:10.
In case the mixture ratio of the fine particle emulsion is larger than 50% by weight, the composition will be the mixture of absorbed particle emulsion and the small particle emulsion. As the result, the composition may have poor blocking resistance. If the mixing ratio is smaller than 5%, the composition will be unstable and have not improved the film-forming property.
Method to know the polymer emulsion with good film forming property in the present invention is observing particle distribution of diluted emulsion freeze-dried by an electron microscope.
Another simple method is comparing particle size distribution with mixed product and raw material emulsion by a laser scattering particle size analyzer.
It is identified that the emulsion composition in this invention is the disappearance and/or the decrease of small particle portions contributed to the fine particle emulsion.
The polymer emulsion of the present invention can be used for producing paper coating composition for making moisture-proof coated paper and/or recyclable-coated paper. The paper coating composition comprises the polymer emulsion, a wax and optionally other ingredients. In the present paper coating composition, the content of the polymer emulsion is usually 60 to 99.5% by weight and the content of the wax is usually 0.2 to 20% by weight.
The wax used in the present invention means water-proof solid substances having 50 to 90xc2x0 C. melting point, becoming liquid phase in the temperature above melting point and containing crystalline structure in solid phase. Examples of the wax include natural waxes such as mineral waxes (e.g. paraffin wax); plant waxes (e.g. carnauba wax) and animal waxes (e.g. bee wax, spermaceti), and synthetic waxes such as polyethylene wax.
The content of the other ingredients (e.g. pigments, filler, water soluble polymers, surfactants and etc.) is usually 0 to 30% by weight.